Hydrodynamic Characteristics of a Novel Annular Spouted Bed with

Three distinct spouting stages have been examined and outlined with the hold-ups increase. In the fully developed spouting stage, three flow behaviors...
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Hydrodynamic Characteristics of a Novel Annular Spouted Bed with Multiple Air Nozzles Xiwu Gong, Guoxin Hu,* and Yanhong Li School of Mechanical & Power Engineering, Shanghai Jiaotong UniVersity, Shanghai, 200030 P.R. China

A novel spouted bed, namely, an annular spouted bed with multiple air nozzles, has been proposed for dryness, pyrolysis, and gasification of coal particulates. It consists of two homocentric upright cylinders with some annularly located spouting air nozzles between inner and outer cylinders. Experiments have been performed to study hydrodynamic characteristics of this device. The test materials studied are ash particle, soy bean, and black bean. Three distinct spouting stages have been examined and outlined with the hold-ups increase. In the fully developed spouting stage, three flow behaviors of particles have been observed and delimited. The effects of nozzle mode and spouting velocity on the maximum spouting height of the dense-phase region, spoutable static bed height, and spouting pressure drop in the bed have been investigated experimentally. Introduction A spouted bed is a kind of high-performance reactor for gassolid or fluid-solid particles reaction. This technology is applied to a wide variety of chemical processes such as dryness, prilling, coating, gasification, combustion, pyrolysis, etc. Compared with the fluidized bed, a spouted bed has several advantages, such as high efficiency of gas-solid contact, big capacity for handling a coarse particle, stable operation in a wide range of gas flow rates, small pressure drop, and little fluidizing air flow rate, etc.1-12 However, due to several limitations of the conventional spouted bed (CSB), such as low annulus aeration and slow solids turnover, which is one of the reasons for the limited commercial use of spouted bed,13 the particles in the bed have a long residence time and poor efficiency of heat and mass transfer. Since the conventional spouted bed suffers from these shortcomings, many modified spouted bed designs have been proposed during the past decades,14 for example, multiple spouted beds,15-17 spouted fluidized beds,18,19 conical spouted beds,20-22 draft tube spouted bed,23-27 pulsed spouted bed,13 and rotating spouted bed.14,28,29 Moreover, the particle hydrodynamic characteristics and gas-solid reaction models in these spouted beds have been studied. Lefroy and Davidson reported the dynamic behavior of the gas phase and solid phase in a spouted bed using a gas-solid flow model.30 Littman et al. applied the Euler equation to forecast the gas-solid flow pattern in a spouted bed.31 San Jose et al. conducted a simulation of the velocity profile and movement trace of a particle in a bed by solving the mass conservation equation.32 Matthew et al. reviewed the residence time and hydrodynamics of a particle of a spouted bed with a draft tube.33 Although many modified spouted beds have been proposed as mentioned earlier, little information on any spouted bed which can be applied to coal pyrolysis and gasification with cogeneration is available. Recently, based on the conventional cylindrical (or square) multiple air nozzle spouted bed, a novel annular spouted bed with multiple air nozzles has been proposed by Shanghai Jiaotong University.34,35 The spouted bed, which combines spouting and coal dryness, pyrolysis, and gasification, is extremely innovative. It is expected to offer good conditions for coal cleaning combustion. The novel annular spouted bed * To whom correspondence should be addressed. Tel./Fax: +8621-62812034. E-mail: [email protected].

consists of two homocentric upright cylinders. The spouting phenomenon takes place in the annular zone between the inner and outer cylinder. Compared to the conventional cylindrical (or square) multiple air nozzle spouted bed, the novel reactor occupies more space due to the fact that it has a hollow central section. However, the particle in the reactor exhibits better quality of solids mixing and flow. Some particles spouted into the hollow central section are collected, and the particles spouted in the annular zone are supplied through the feed regulator. Also, the device can remain in continuous operation and the spouted particles would be replaced at a certain control operation condition. Therefore, it is very necessary to study particle hydrodynamic characteristics in the spouted beds. The aim of this paper are to (1) observe the pattern of solids movement and gain understanding of the flow regimes in which the bed evolves from the emptied bed into the fully developed spouting state, (2) discuss the effects of nozzle mode and velocity as well as particle size, shape, and density on the maximum spouting height of dense-phase and spoutable static bed height, and (3) investigate the effect of increasing or decreasing air flow on pressure drop in the bed. Results are reported for various particulate materials under various operating conditions. Experimental Setup, Materials, and Methods Experimental Setup. A schematic diagram of the experimental setup and the associated instrumentation is shown in Figure 1. The diameters of two homocentric upright pellucid methyl-methacrylate cylinders are 300 and 400 mm, respectively. In addition to the annular spouted bed, the experimental setup consists of an air blower, a gate valve, a rotating cone, an oscillating feeder machine, an electromotor, a data acquisition system, and some discharge air pipes. The spouting air supplied by the air line is regulated by a gate valve. For the purpose of the hydrodynamic study, the experiments were run with air at room temperature and atmospheric pressure; only one inlet air temperature, 20 °C, is used throughout. The oscillating feeder machine and rotating cone work together to feed the test materials into the novel annular spouted bed. The data acquisition system is made up by some pressure sensors, a serial port, and a personal computer. Eight nozzles are mounted uniformly along the circumference on the bottom of the annular space between the inner and outer cylinders. To compare the effects of different nozzle modes on the hydrodynamic characteristics

10.1021/ie051381y CCC: $33.50 © 2006 American Chemical Society Published on Web 05/18/2006

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Figure 1. Schematic diagram of the overall experimental setup. 1 air blower; 2 gate valves; 3 air room; 4 nozzle; 5 V-shape air splitter; 6 spouted bed; 7 rotating cone; 8 exhaust tube; 9 oscillating feeder machine; 10 electromotor; 11 pressure sensor; 12 pitot tube; 13 I/O board; 14 port; and 15 PC.

Figure 2. Schematic drawing of two nozzle structure. Table 1. Dimension of Particles Used in the Experiments14 material

L (mm)

B (mm)

Z (mm)

Dpe (mm)

Dpgm (mm)

Dp (mm)

φ

ash particle soy bean black bean

5.453 6.968 9.148

3.251 6.604 7.808

3.186 6.092 5.952

4.372 8.121 9.331

3.836 6.545 7.519

3.074 7.626 7.670

0.703 0.939 0.822

Table 2. Physical Properties of Bed Particles Used in the Experiments14 material

Fs (kg m-3)

Fb (kg m-3)

Ar



ash particle soy bean black bean

1486 1228 1183

773 616 605

4.866 × 105 1.806 × 107 1.914 × 107

0.480 0.498 0.489

of the particles, two different nozzle modes were adopted: (1) angled and (2) forward nozzle structures. The schematic drawing of the two nozzle structures are shown in Figure 2. On the nozzle, a V-shaped air splitter is arranged to prevent dead zone formation. Its height is 130 mm. It is a well-documented fact that the multiorifice spouted beds are not easy to operate properly because certain orifices may collapse, and consequently, dead zones are formed in the bed. To avoid this problem, an independent inlet for each orifice has been proposed in the literature.15,17 In the design of the novel annular spouted bed, an air room is connected under the bed. The spouting air is sent to the air room through an inlet, 100 mm in diameter, which is located at the center of the room base. Then, the air was divided equivalently into eight nozzles, which is helpful

to develop uniform spouting without partial choking of the nozzle. Compared to the bed with an independent inlet for each orifice, the novel spouted bed has simpler construction. The bed was prepared in a standardized manner in order to minimize any variation that may arise due to residual particles from a previous test during the feeding of the test materials. This was done by emptying the bed before starting each experiment. To minimize the electrostatic effect in the system, both the inner and outer walls of the column were sprayed with an antistatic agent before starting each experiment. Materials. Ash particle, soy bean, and black bean were used as the test materials in this study. According to Geldart’s classification,36,37 all particles used in this work belong to group D (spoutable, large, and dense particles). Dimensions and physical properties (average values) of the particles tested are given in Tables 1 and 2. The particle dimensions, namely, length (L), width (B), and thickness (Z), were measured using a dial caliper having a least count of 0.02 mm. Two hundred particles were measured for each test material. The bulk density, Fb, was determined by pouring a weighed amount of sample particles through a funnel into a measuring cylinder. The volume occupied by the sample was then used to determine the value of the loose bulk density. The particle density, Fs, was computed using fluid displacement with water as measuring fluids. An electronic balance accurate to (0.001 g was used for weight determination. Methods. After the air blower is powered, the spouting air is blown into the air room first. Then, it spouted into the annular space of spouted bed through eight nozzles. Subsequently, the oscillating feeder machine begins to offer the rotating cone with test materials. With the help of the centrifugal action produced by the rotating cone driven by an electromotor, the test material moves upward along the wall of cone, escapes the cone, and then falls into the annular space between the inner and outer cylinders. These particles are lifted and suspended by the air flow in the annular space. The air flow will be discharged by the exhaust tube on the top of the spouted bed. The ranges of experimental conditions employed are as follows: the rotational speed of cone N ) 6 Hz and total air flow rate Q ) 0-960 m3/h. The studied experimental conditions for each item are given in Table 3. In normal conditions the dependent variables measured are the maximum spouting height of dense-phase H2, the maximum spoutable static bed height H, the minimum spouting air flow rate Qms, and bed pressure drop ∆P. Usually, the resistance to spouting air in the bed increases as the hold-ups in the bed are increased. When the hold-ups in the bed are equal to the maximum spoutable amount W for a certain air flow rate, the oscillating feeder machine will be stopped immediately. The total air flow rate at this point is noted as the minimum spouting flow rate, Qms. The spouting height of the dense phase is measured by the staff guage mounted outside the spouted bed. The air flow rate or nozzle spouting velocity is measured with a precalibrated pitot tube. Differential pressure drop across the pitot tube is measured using a manometer. Pressure drop across the bed is measured using static pressure taps connected to some calibrated differential pressure sensors. The analogue signals for various sensors are acquired by a data acquisition system

Table 3. Studied Experimental Conditions item

material

total air flow (m3/h)

nozzle

temp (°C)

bed height (static)

flow regime height of dense-phase region minimum spouting condition pressure drop

soy bean ash particle, soy bean, black bean ash particle, soy bean, black bean soy bean

960 0-960 0-960 0-960

angled angled or forward angled or forward angled

room temp room temp room temp room temperature

0.2 or 0.3 m

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Figure 3. Particles flow behaviors observed for the annular spouted bed with multiple air nozzles (Q ) 960m3/h, soybean).

connected to a personal computer for digitized data gathering. The air blower will be shut up after the test jobs mentioned above are finished. Finally, the static bed height and weight for hold-ups in the bed will be measured. The bed pressure drop is calculated according to the method of Mathur and Epstein38 as follows

∆Pbed ) x(∆Ptot2 - ∆Pemp2)

(1)

The minimum spouting velocities based on the equivalent cylindrical column and nozzle diameters are calculated from the total air flow rate, Qms, using the following expressions

Umsec )

Umsn )

4Qms πDec2 4Qms πDn2

(2)

(3)

where Dec and Dn are the equivalent cylindrical column and nozzle diameters, respectively. The measurement uncertainty values due to random errors are the ones supplied by the manufacturers or are estimated from a ‘single-sample’ experiment.39-41 The rule of thumb in the latter case is that the maximum possible error is equal to plus or minus one-half the smallest scale division (the least count) of the instrument. The precision grade of the pressure transducer is 0.2. A staff guage accurate to (0.001 m is used for height determination. To investigate the reproducibility of the results in the experiment, replicates are made of randomly selected experiments. From these tests the reproducibility values for the maximum spoutable bed height H, the minimum spouting air flow rate Qms, and bed pressure drop ∆P were within (7.3%, 6.4%, and 11.7% of their respective reported values. Results and Discussion Flow Regime Diagrams. The flow regime of particles in the annular spouted bed with multiple nozzles is different from that of the conventional spouted bed. Figure 3 shows the observed flow behavior of particles with the hold-ups increasing for the annular spouted bed with adopted angled nozzle. Soy bean is the test material. Three distinct spouting stages are obtained with increasing hold-ups. When there are fewer hold-ups in the spouted bed, the particles can be spouted out of the V-shaped air splitter and begin to oscillate and circulate in annular space, 45 cm in height. With the angled nozzle and V-shaped air splitter, particles cannot exist around the nozzle. This spouting initial stage is shown in Figure 3a. With increasing hold-ups, the weight of hold-ups

will offset the driving force of air gradually. When the holdups reach a certain value, the particles will begin to accumulate and pack along the air splitter on the nozzles. The packed moving height of particles increases as the hold-ups are increased until its height is equal to the height of the air splitter. This process is called the spouting developing stage, as shown in Figure 3b. As the hold-ups increase further, the driving force of air cannot conquer the bed pressures and the spouting air cannot spout directly to the surface of the particles in the bed. As a result, the dense-phase spouted fluidizing region is above the packed moving region. This flow regime of particles is fluidizing, bubbling, or slugging. Due to the limitation of air splitter, the height of the packed moving region is unchanged with increasing hold-ups in the bed while the height of the dense-phase spouted fluidizing region increases gradually. At the surface of the dense-phase spouted fluidizing region some particles driven by the air bubbles will be spouted upward intermittently about 10 cm in spouting height. These particles form the dilute-phase entrainment region at the upper space of the bed. Hence, along the direction of the bed height, as shown in Figure 3c, the flow regime of particles in the bed can be divided into three different regions: the packed moving region in the V-shaped air splitter, the dense-phase spouted fluidizing region on the V-shaped air splitter, and the dilute-phase entrainment region at the upper part of the annular space. The zonal phenomenon will remain unchanged until the hold-ups in the bed reach the maximum spoutable amount. This process is called the fully developed spouting stage. If the hold-ups exceed the maximum spoutable amount, the particle bed becomes unstable for some nozzles blocked by the packing materials. The bed cannot be fluidized at such operating conditions. Maximum Height of the Dense-Phase Spouted Fluidizing Region. Figure 4 shows a plot of the maximum height of the dense-phase spouted fluidizing region versus the nozzle spouting velocity. The vertical ordinate shows the maximum height of the dense-phase spouted fluidizing region for the maximum hold-ups corresponding to the nozzle spouting velocity. For all particles the maximum height of the dense-phase spouted fluidizing region increases as the nozzle spouting velocity increases. It can be explained that the driving force of air is strengthened with the increase of nozzle spouting velocity when the hold-ups remain unchanged; so, the particles will be spouted to a higher position. As a result, the maximum spouting amount of a high nozzle spouting velocity is bigger than that of a low nozzle spouting velocity. The greater space is occupied by the particles for a high nozzle spouting velocity, resulting in a higher bed height. As observed from Figure 4, at the same nozzle spouting velocity, the height of the dense-phase spouted fluidizing region for the larger particles is higher than the smaller particles. The explanation is that the density of larger particles used in the experiment is lower; so, the height of the dense-phase spouted fluidizing region for larger particles is higher at the same nozzle spouting velocity. Comparing Figure 4a with Figure 4b, at the same height of dense-phase spouted fluidizing region, the nozzle spouting velocity of the forward nozzle is smaller than that of the angled nozzle. The spouting air has a higher angular velocity component at a higher nozzle spouting velocity for the angled nozzle as a result of deviation from the vertical spouting velocity. Since only the vertical velocity component sustains the spouting by offsetting the gravitational effect, the particles for the forward nozzle are spouted upward easily. Therefore, the height of the

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Figure 4. Effects of nozzle minimum spouting velocity and particle species on maximum height of dense-phase spouted fluidizing region.

dense-phase spouted fluidizing region is higher than that of the angled nozzle. Minimum Spouting Velocity and Maximum Spoutable Static Bed Height. Figure 5 presents an example of the variation of dimensionless bed height H/Dec with Rems for different particles. For all particles, the static bed height increases almost linearly as the Ums and hence Rems are increased. The deeper the bed, the larger the amount of air needed to produce spouting condition and therefore the higher Ums. Similar observations are also reported in the literature for spouted beds of other designs.29 The test materials for spouting and fluidizing in particles bed is subjected to the driving force upward of air and gravity downward mainly. Because the smaller sized particles lead to a larger ratio of mass to surface area, the driving force per unit mass particle will increase with decreasing particle size. Thus, the maximum spoutable amount of the smaller particles is larger. Namely, the maximum spoutable static bed height of small size particles is higher than that of large size particles, and the minimum spouting velocity for small sized particles is less than that of large sized particles under the same spouting condition. The spouted bed with a forward nozzle can spout a deeper bed of particles than that with an angled nozzle, i.e., a higher air flow rate is needed for the angled nozzle to get the minimum

Figure 5. Effect of RemsnRemsec on dimensionless bed height.

spouting state at the same static bed height. This is attributed to the same reason mentioned earlier. Additional information on the minimum spouting air flow requirement of the annular spouted bed is obtained by examining the effect of particle properties, namely, diameter, shape, and density. These properties, along with the fluid properties (density, viscosity), are often combined into a single parameter, namely, the Archimedes number, Ar (which represents the ratio of the gravity force to viscous force). The effect of this parameter on Rems or the dimensionless bed height is illustrated in Figure 5 for a different nozzle model. The main trend is for both Rems and the dimensionless bed height to increase with Ar. A possible explanation is that a material with larger Ar, in this case black bean, offers more resistance to flow than does a material with lower Ar. Larger and denser particles respond more slowly to the change in fluid flow rate, while smaller and lighter particles follow more closely the changes in fluid motion. Therefore, gravity is more significant than viscous forces for the larger particles under spouting conditions.14 Spouting Mechanism. Generally, the spouting mechanism in which the bed of particles changes from a packed state to

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Figure 6. Effect of Re on bed pressure drop for soy bean in spouted bed with angled nozzle.

fully developed spouting is best described with reference to plots of bed pressure drop vs superficial air velocity. To generalize the analysis, the pressure drop and superficial air velocity are written in dimensionless form, i.e., ∆P/(FbgH) and Re. The former represents the ratio between the bed pressure drop and the bed weight per unit cross-sectional area, while the latter represents the particle Reynolds number. Figure 6 shows typical dimensionless pressure drop evolution traces from a packed state to a fully developed spouting for soy bean in the bed with static height 0.2 and 0.3 m and an angled nozzle. The plots are supplemented by branches illustrating the reverse process, that is, the collapse of spouts upon decreasing air flow rate. The following sequence of events is observed as the air flow rate is increased or decreased.29 Experimentally, the gate valve was gradually opened until an overdeveloped spout was formed. After steady spouting, the gate valve was slowly closed and the air flow rate gradually reduced until the spout just collapsed. Little reduction of air velocity at this condition causes the spout to collapse and the pressure drop in the bed to rise suddenly. The air spouting velocities at this point were defined as the minimum spouting

velocity (Ums), depending on solid and fluid properties on one hand and bed geometry on the other. Experimental values of Ums are determined at the point where the spouts collapse as the air flow rate is decreased.14,29,42 As shown in Figure 6, the pressure drop is similar to that of the fluidized bed. The ranges of experimental conditions employed were H ) 0.2 or 0.3 m, Q ) 0-960 m3/h, and ∆P ) 0-3.5 kPa. Increasing Air Flow. As in conventional spouted beds, the pressure drop increases almost linearly with increasing air flow rate while the fixed-bed state remains unchanged. Some internal annular cavities are formed along the direction of nozzle spouts. Some arch of compacted particles that offers high resistance to flow exists above the internal spouts so that the pressure drop rises until it reaches a peak value at point B. Beyond point B the height of the internal spouts increases and the air has enough momentum to pierce the bed surface; hence, spouting begins. Therefore, the pressure drop suddenly decreases to point C. After point C, both spouts are fully developed and the pressure drop remains nearly constant. Decreasing Air Flow. While the bed remains in the fully developed spouting state, the air flow rate decreased, the pressure drop remains nearly constant, and the bed remains in the spouting state until point C′ is reached; this represents the minimum spouting condition (Ums or Rems). A slight reduction of the air flow rate causes the spouts to collapse and the pressure drop to rise suddenly to point B′. Subsequently, the bed pressure drop decrease with the air flow rate is decreased. It was observed throughout the two processes that the bed pressure drop remains nearly constant with increasing or decreasing nozzle spouting velocity if the bed remains a fully developed spouting state. For a given bed height and air nozzle area, the minimum spouting velocity and pressure drops of the novel annular spouted bed seems to be higher than that of a conventional spouted bed. In a conventional spouted bed the gas from the spout diffuses out through the cylindrical area of the spoutannulus interface, whereas in the case of the annular spouted bed the gas from the spout diffuses out through cylindrical areas of the spout-inner core interface and spout-annulus interface. The higher area of gas diffusion into the solid bed requires more flow to counter diffusion out of the spout, so that the spout does not collapse. For a given air nozzle area, the amount of outward gas flow from the spout of the annular spouted bed is higher than that of the conventional spouted bed because of the higher interface area of the annular spouted bed. As a result, the packed bed of the annular spouted bed reaches the minimum spouting condition at a lower bed height.42 Conclusions In this paper a modified spouted bed, namely, an annular spouted bed with multiple air nozzles, was developed. Experiments have been performed to study the hydrodynamic characteristics of this device. The following conclusions are drawn for the present investigation. (1) The novel annular spouted bed with multiple air nozzles has some advantages. The volume and mass of particles spouted by air are increased obviously. Three flow stages for the particle bed are obtained successively starting from the hold-ups at zero and gradually increasing to the maximum spouting weight, namely, spouting initial stage, spouting developing stage, and fully developed spouting stage. (2) For all test particles, the maximum height of the densephase spouted fluidizing region increases as the nozzle spouting velocity increases. At the same nozzle spouting velocity, the height of the dense-phase spouted fluidizing region for the large sized particles is bigger than that of the small sized particles.

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The particles are spouted upward more easily for the spout bed with some forward nozzles, resulting in a larger height of the dense-phase spouted fluidizing region. (3) Similar to the conventional spouted bed, the static bed height increases almost linearly as Ums and hence Rems are increased. The annular spouted bed with a forward nozzle can spout a deeper bed of particles than that with an angled nozzle, i.e., a higher air flow rate is needed for the angled nozzle to get the minimum spouting state at the same static bed height. The minimum spouting velocity of a material with larger Ar is bigger than that of a material with lower Ar. (4) The bed pressure drop remains nearly constant with increasing or decreasing nozzle spouting velocity if the bed remains a fully developed spouting state. The given results show the hydrodynamics of the novel annular spouted bed with multiple air nozzles at cold conditions. Further experiments are required to fully understand the behavior of this newly developed spouted bed. Acknowledgment The authors gratefully thank financial support by the National Science Foundation of China under grant no. 50376033. Nomenclature B ) breadth (m) Dp ) effective particle diameter (m) ) DpeΦ Dpe ) equivalent spherical diameter (m) Dpgm ) geometric particle diameter (m) ) (LBZ)1/3 Dec ) equivalent cylindrical column diameter (m) Dn ) nozzle diameter (m) g ) gravitational constant (m s-2) H ) spoutable height (m) H1 ) packed moving height (m) H2 ) dense-phase spouted fluidizing height (m) H3 ) dilute-phase entrainment height (m) L ) length (m) N ) rotational speed of rotating cone (Hz) ∆Pbed ) bed pressure drop (kPa) ∆Pemp ) empty annulus pressure drop (kPa) ∆Ptot ) total bed pressure drop (kPa) Q ) total air flow rate (m3 s-1) U ) velocity (ms-1) W ) hold-up (kg) Z ) thickness (m) Greek letters  ) voidage (1 - (Fs/Fb)) Fb ) bulk density (kg m-3) Fs ) particle density (kg m-3) Fg ) fluid density (kg m-3) Φ ) sphericity ) Dpgm/L µg ) fluid viscosity (kg m-1 s-1) Subscripts and superscripts ec ) equivalent cylindrical column n ) nozzle ms ) minimum spouting condition msec ) equivalent column minimum spouting condition msn ) nozzle minimum spouting condition Dimensionless groups Ar ) Archimedes number, ([Dp3Fg(Fs Re ) Reynolds number, (DpUFg/µg)

-

Fg)g]/µg2

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ReceiVed for reView December 12, 2005 ReVised manuscript receiVed April 14, 2006 Accepted April 19, 2006 IE051381Y